1. Field of the Invention
The present invention relates to a waveguide and a thermally assisted type magnetic head using this waveguide.
2. Description of the Related Art
Recently, in magnetic recording devices, such as hard disk devices, performance improvement of a thin film magnetic head and a magnetic recording medium is in demand in association with high recording density. As the thin film magnetic head, a combination type thin film magnetic head is widely used, where a reproducing head having a magneto resistive effect element (hereafter, also referred to as MR element) for reading, and a recording head having an inductive transducer (magnetic recording element) for writing are laminated onto a substrate. In a hard disk device, the thin film magnetic head is disposed in a magnetic head slider that flies slightly above the surface of the magnetic recording medium.
A magnetic recording medium is a discontinuous medium where magnetic micro particles are assembled, and each magnetic micro particle has a single-domain structure. In this magnetic recording medium, one recording bit is composed of a plurality of the magnetic micro particles. In order to enhance the recording density, asperity of the boundary between adjacent recording bits has to be small, which means that the magnetic micro particles have to be small. However, if the magnetic micro particles are decreased in size, thermal stability of magnetization of the magnetic micro particles is reduced. In order to solve this problem, it is effective to increase anisotropic energy of the magnetic micro particles. However, if the anisotropic energy of the magnetic micro particles is increased, coercive force of the magnetic recording medium becomes great and it becomes difficult to record information by the existing magnetic head. Such a problem exists in conventional magnetic recording, which is a major obstacle to increasing the recording density.
As a method for solving this problem, the method of so-called thermally assisted magnetic recording is proposed. In this method, a magnetic recording medium having great coercive force is used, and when recording information, a magnetic field and heat are simultaneously applied to a part of the magnetic recording medium where information is recorded. This causes a rise in temperature in the part where the information is recorded and a reduction of the coercive force, and information is then recorded.
In the thermally assisted magnetic recording, a method using near field light is a known technique to add heat to a magnetic recording medium. Near field light is a type of so-called electromagnetic field to be formed around a substance. Normal light cannot be tapered to within a region that is smaller than the wavelength due to diffraction limitation. However, irradiation of light with the same wavelength on the microstructure causes the generation of near field light depending upon the microstructure scale, and enables light to be sharply tapered within a minimal region such as several tens of nm. As a specific method to generate the near field light, a method to generate laser light by a laser diode and to generate the near field light by a plasmon excited by the laser light is commonly known. The near field light is generated with a metal referred to as a probe, which is a so-called plasmon antenna.
In the plasmon antenna, the near field light is generated by directly irradiating light to the antenna, but in this technique, a conversion efficiency of the irradiated light to the near field light is low. A majority of the light energy irradiated to the plasmon antenna is reflected by the surface of the plasmon antenna or converted into thermal energy. Since the size of the plasmon antenna is set at or less than the wavelength of the light, the volume of the plasmon antenna is small. Thus, in the plasmon antenna, the temperature increase in association with the heat generation becomes very significant.
Such a temperature increase causes the plasmon antenna to expand in volume and to protrude from an air bearing surface, which is a surface facing the magnetic recording medium. The end part positioned on the air bearing surface of the MR element is away from the magnetic recording medium, and as a result, there is the problem that a servo signal recorded in the magnetic recording medium cannot be read at the time of recording movement.
Therefore, a technology where light is not directly irradiated to the plasmon antenna is proposed. For example, in a technology is disclosed in the specification of U.S. Pat. No. 7,330,404, propagating light that enters from the laser diode and that has propagated through a core of waveguide, such as a fiber optic element, is combined with a plasmon generator via a buffer portion in a surface plasmon polariton mode, and the surface plasmon is excited in the plasmon generator. The plasmon generator has an edge of plasmon generator that is positioned on the air bearing surface and that generates a near field light and a propagation edge facing the waveguide via the buffer portion. The light propagating through the core is totally reflected on the interface between the core and the buffer portion, on which occasion, light that penetrates toward the buffer portion, referred to as evanescent light, is generated. This evanescent light and a collective vibration of electrical charges in the plasmon generator are combined, and the surface plasmon is excited in the plasmon generator. The excited surface plasmon propagates to the edge of plasmon generator along the propagation edge and generates the near field light at the edge of plasmon generator. According to this technology, since light that propagates through the waveguide does not directly irradiate the plasmon generator, an excessive increase in temperature of the plasmon generator can be prevented.
Meanwhile, a laser diode for generating a laser light is arranged separately from a magnetic head slider. For example, in the specification of U.S. Pat. No. 7,643,248, a configuration is disclosed in which a surface emitting laser diode is disposed separately from a magnetic head slider. However, when the laser diode is independently disposed, a process to connect a magnetic head slider and the laser diode is required. In the connection process, it is required that a light outgoing part of the laser diode be connected to a core disposed in a magnetic head slider with high positioning accuracy. Since the laser diode is arranged in an exposed manner, it is also not preferable from a reliability standpoint. Therefore, a configuration to incorporate the laser diode in the magnetic head slider has been desired.
The simplest configuration where the laser diode is incorporated in the magnetic head slider is obtained when the laser diode is disposed such that an outgoing surface of the laser diode faces the air bearing surface of the magnetic head slider, such that an outgoing direction of the laser light is in a direction orthogonal to the air hearing surface. Since the laser light is transferred to the vicinity of the air bearing surface in a straight-shaped core as propagating light in this configuration, the propagating loss of the propagating light is small.
However, it is difficult to dispose the laser diode on the actual magnetic head slider in such a manner. FIG. 1A is a conceptual view of the laser diode disposed in an ordinary magnetic head slider in a direction where the outgoing direction of the laser light is orthogonal to the air bearing surface. Specifically, FIG. 1A is a cross sectional view of the magnetic head slider cut in the vicinity layer of the recording head. A plane size of a magnetic head slider 1 for a femto magnetic head slider, which is currently conventional, is approximately 230 μm (longitudinal direction size)×approximately 700 μm (transversal direction size), and a side including the air bearing surface S is defined as a longitudinal side. On the other hand, depending on an output of the laser light, the laser diode 31 should have a size whose side parallel to the outgoing direction of the laser light is a length of 300 μm or more and whose side orthogonal to the other side is a length of, for example, 120-200 μm. Moreover, a spot diameter of the laser light output from the laser diode is approximately 4 μm; on the other hand, the diameter in an adjacent part to a plasmon generator 16 should be tapered to approximately 0.5 μm. Therefore, the core should provide a cross section narrowing part 15d where a cross section gradually narrows along a propagating direction of propagating light. Since it is impossible to drastically vary the cross section of the cross section narrowing part, the cross section needs a length of approximately 100 μm in the propagating direction of the propagating light in order to taper the spot diameter from approximately 4 μm to approximately 0.5 μm.
Considering the matter described above, when the laser diode is arranged in a direction where the outgoing direction of the laser light is orthogonal to the air bearing surface, a length of at least 400 μm in a direction orthogonal to the air bearing surface is required for the laser diode 31 and the cross section narrowing part 15d. Additionally, since a space for disposing the plasmon generator 16 is also required, it is completely impossible to incorporate the laser diode with a conventional magnetic head slider. As illustrated with broken lines in the drawing, a size that is at least approximately twice that of the conventional longitudinal direction size is required.
On the other hand, as illustrated in FIG. 1B, when the laser diode 31 is arranged in a direction where the outgoing direction of the laser light is parallel to the air bearing surface, since there is a marginal space in the lateral direction of the magnetic head slider, it is possible to incorporate the laser diode with the conventional magnetic head slider. However, in such a configuration, a direction of a waveguide should be curved 90° in the middle as illustrated in the drawing. Technology that enables the waveguide to be curved in the middle is conventionally known. However, such a curvature radius is generally large, and technology that can be applied to a micro structural body, such as a magnetic head slider, with a scale of 0.1 mm or smaller is not known.
A waveguide having a curved part is disclosed in Japan Laid-Open Patent Publication No. H11-125726; however, the curvature radius of the waveguide is approximately 25 mm. A waveguide having an S-shaped curved part is disclosed in Japan Laid-Open Patent Publication No. H11-167032. The waveguide has a core that has a semicircular cross section arranged on an upper surface of a cladding part. In the vicinity of the core, a reflection groove is formed extending along a path of the core. The upper surfaces of the core and the reflection groove are opened and are not covered by the clad. Adjustment of the interval between the core and the reflection groove suppresses propagating loss and makes the curvature radius of the curved part small. However, a practical curvature radius is approximately 50 mm. A waveguide providing a curved part is disclosed in Japan Patent No. 4202212 as well. The curved part is formed by continuously connecting multiple minute curved lines having different respective curvature radii. However, the minimum curvature radius is approximately 5 mm, and the substantial curvature radius of the curved part is larger than the radius.
As described above, the curvature radius of the conventional curved waveguide is formed in the order of mm. However, when the curved part is disposed in the waveguide of the magnetic head slider incorporating the laser diode, a waveguide of the conventional art that realizes only a curvature radius on the order of mm cannot be applied as shown in FIG. 1B. When the curved waveguide is applied to the magnetic head slider, the curvature radius must be on the order of at least 10 μm. Considering that the magnetic head slider will be further miniaturized in the future, a waveguide having a curvature radius of 10 μm or less is preferably required.
An object of the present invention is to provide a waveguide that can propagate laser light from the laser diode as propagating light, that is curved in one direction, and that has a curvature radius that is substantially reduced. Further, another object of the present invention is to provide a magnetic head of a thermally assisted magnetic recording system in which such a waveguide is used.